In this introductory section on perception, we will review the
concept of modality from the neurobiological point of
view [153,308], gradually narrowing the scope to those
modalities relevant to research within .

As remarked by Shepherd [308], the notion of sensory modality can be
traced back to the 1830s, in particular to the monumental ``Handbook of
Human Physiology'', published in Berlin by Johannes Muller, who promulgated
the ``law of specific nerve energies''. This states that we are aware not
of objects themselves but of signals about them transmitted through our
nerves, and that there are different kinds of nerves, each nerve having its
own ``specific nerve energy''. In particular, Muller adopted the five
primary senses that Aristotle had recognized: seeing, hearing, touch,
smell, taste. The specific nerve energy, according to Muller, represented
the sensory modality that each type of nerve transmitted.

The modern notion, beyond a great degree of terminological confusion, is
not very much different: we recognize that there are specific receptor
cells, tuned to be sensitive to different forms of physical energy in the
environment and that they serve as stimuli for the receptor cells. A table
in Shepherd's book illustrates the point. The table can be simplified and
re-written in our framework as follows (Table 2.1 ).

Table 2.1
: An overview of input channels at the neurophysiological level

The different sensory modalities used by human beings are not processed in
isolation. Multimodal areas exist in cortical and sub-cortical areas, such
as the posterior parietal cortex (area 5 and 7) and the superior
culliculus. The integration of the different channels is essential, among
other things, for allowing the brain to reconstruct an internal body model
and an internal representation of external Euclidean space [228].